Pressurized water nuclear reactors have a core consisting of assemblies arranged vertically and side-by-side in a vessel containing pressurized water which is responsible for cooling the core and transporting the heat from this core to the steam generators. The steam provided by the steam generators makes it possible to drive a turbine which is itself responsible for driving a turbo-alternator producing electric current.
Depending on the needs of the electricity network and on the manner in which the power station is coupled to other nuclear power stations, it is necessary to ensure the control of the nuclear reactor to obtain from this reactor at any time a power corresponding to the power demanded.
The control of the reactor is usually ensured by the vertical displacement, inside the core, of control rods which absorb the neutrons. In the most frequent modes of control of a pressurized water nuclear reactor, the movement of a control group consisting of highly absorbing rods is controlled automatically by employing as a regulating parameter the deviation between the real mean temperature of the core and a reference temperature which is a linear function of the power which the nuclear reactor has to supply to the turbine. However, in all the modes of control of a pressurized water nuclear reactor, it is necessary to have available an additional means for controlling the nuclear reactor. This additional means consists of a system for boration and dilution of the reactor cooling water, i.e., a system of means making it possible to vary the soluble boron content of the nuclear reactor cooling water, which can be introduced in the form of boric acid or, on the contrary, diluted by introducing pure water. The increase in the soluble boron content makes it possible, in fact, to increase the absorption of the neutrons by the cooling fluid and hence to reduce the reactor power. The dilution obviously has an opposite effect.
The system for boration and dilution of the reactor cooling water makes it possible to complement the action of the groups of control rods and, in particular, to correct the long-term effects due to the variations in reactor reactivity. These long-term effects which accompany the variations in reactor reactivity include in particular the formation and the disappearance of xenon through a nuclear reaction in the reactor core. The appearance and the conversion of xenon have, in themselves, a major influence on the reactivity and on the axial distribution of power in the reactor core.
The axial distribution of power in the reactor core, i.e. the distribution of power in the vertical direction, is in fact neither homogeneous nor constant for a variety of reasons, the main ones of which are that the control rods employed for controlling the reactor are generally inserted only over a part of the height of the core, that this insertion varies over time, and that the density of the cooling water and the concentration of xenon in the reactor core are not constant along its height.
One of the objectives sought after in the course of reactor control is to avoid the power distribution in the core being excessively imbalanced between the upper part and the lower part of the core.
To control the axial distribution of power in the core, there are usually available means for measuring the neutron flux at various heights in the core and means for calculating a parameter expressing the imbalance of power in the core, called "axial power inbalance .DELTA.I" and defined as follows: EQU .DELTA.I=P.sub.H -P.sub.B
where P.sub.H is the power in the high half of the core, P.sub.B the power in the low half of the core and .DELTA.I the axial power imbalance expressed as a percentage of the nominal power.
More precisely, a determination is made of the deviation in the value of the axial imbalance measured relative to a reference axial power imbalance .DELTA.I ref which corresponds to the value of the .DELTA.I measured at 100% of the reactor power, the control rods being virtually drawn out and the xenon being at equilibrium throughout the reactor core.
In addition to the regulation of power with control rods comprising, in particular, the movement of a control group as a function of the deviation in the core temperature relative to a reference temperature, there is in operation a manual control of the boron content of the cooling water to make it possible to conform to the instructions for positioning the control rods as a function of the reactor power level. These positioning instructions are established so as to maintain the deviation in the power imbalance relative to the reference imbalance in a zone of small amplitude around the zero value.
The means for boration and the means for dilution are controlled manually by an operator. This partly manual mode of control can be considered as relatively satisfactory in the case where the power station is employed at a constant power level or with very slow variations in power levels.
During an operation of the power station following loading, when the variations in power are more numerous and faster, the necessary actions of boration or dilution are most frequent. It can then be very useful to have available a means of automatic control of the actions of boration and dilution.
In U.S. Pat. No. 3,570,562, a part of the primary cooling water is diverted continuously into a measuring assembly permitting the momentary boron concentration in this primary water to be determined. The boron concentration is adjusted automatically by virtue of a regulator, as a function of the power demanded from the reactor, the permitted boron concentration limits, and the position of the control rods in the reactor core.
Such a system of automatic regulation is, however, complex because it requires the continuous determination of the boron concentration in the cooling fluid and the establishment of a correlation between the required value of the concentration and the value of various control parameters of the reactor.
In French Pat. No. 2,392,472, there is described a system for automatic boration and dilution based on the comparison between the mean temperature of the core and the reference temperature representing the power demanded by the turbine from the nuclear reactor.
In the case of a mode of reactor control such as described in French Pat. No. 2,395,572 owned by the present assignee, methods of automatic control of the boron content such as described above would not be applicable.
In the control process described in French Pat. No. 2,395,572, groups of control rods having reduced anti-reactivity are moved within the core, only as a function of the power demanded from the turbine. A group of highly absorbing control rods which is different from the power regulation groups is moved as a function of the deviation between the mean temperature of the core and the reference temperature. This temperature regulation group is moved in a manner which is totally independent of the power regulation groups, between control boundaries defined by the operating mode of the reactor and the state of change in the reactor core. To maintain the regulating group between these control boundaries, the concentration of boron in the primary fluid is varied either manually or automatically when the regulating group moves to reach one of the control boundaries.
There is therefore no automatic regulation of the concentration of soluble boron in the cooling water so long as the regulating group does not move so as to reach or go beyond these control limits.
Furthermore, the axial power distribution which is disturbed to a lesser degree than in other modes of control by the power regulating groups, is not controlled automatically.